1. Field of the Invention
The present invention relates to a vacuum casting method of the type wherein a mold cavity is reduced in pressure to a vacuum and upon opening a gate, a molten metal is charged into the cavity at a high speed. More particularly, the present invention relates to an improved vacuum casting method in which bubbles and solid metal pieces are prevented from being involved in the molten metal charged into the cavity.
2. Description of the Related Art
As one example of a light alloy casting method of a high quality and a low cost, a vacuum casting method (named by the present applicant as a Vacuum Precharged Closed squeezed casting method) was proposed by the present applicant in Japanese Patent Application No. HEI 4-309534 filed on Oct. 23, 1992.
In the proposed vacuum casting method, a mold cavity is shut off from an interior of a molten metal retaining dome by a gate. Then, the cavity is reduced in pressure to a vacuum, and substantially simultaneously a portion of a molten metal held in a molten metal holding furnace is raised to the molten metal retaining dome. Then, the gate is opened so that the molten metal in the molten metal retaining dome is charged into the cavity at a high speed due to the vacuum in the cavity. The cavity is shut off by a shut pin, and then the molten metal in the cavity is pressurized by inserting a pressure pin into the cavity. Then, the molten metal in the cavity is cooled to be solidified.
In the proposed vacuum casting method, since the mold cavity is reduced in pressure to a vacuum before the molten metal is charged, the molten metal in the cavity has few or no bubbles, so that casting defects due to bubbles are avoided and casting quality is improved. Further, because of the vacuum generated in the cavity, the charging speed of the molten metal is very high, so that the molten metal can smoothly run in the cavity and, as a result, slimmer and lighter cast products is possible.
However, some additional problems remain in the above-described vacuum casting method. For example, during repeating the casting cycles, solid metal pieces generated in the previous casting cycles, which may be additionally oxidized, often adhere to an inside surfaces of a stalk connecting the molten metal retaining dome and the molten metal holding furnace and/or the molten metal retaining dome. When the molten metal is raised through the stalk to the molten metal retaining dome, the rising molten metal often involves air formed at the solid metal pieces adhering to the surface. The involved air may be suspended in the molten metal as bubbles without floating up to the upper surface of the molten metal. When the gate is opened and a portion of the molten metal located in the vicinity of the gate is charged into the cavity, the bubbles suspended in the molten metal and the solid metal pieces detached from the inside surfaces of the stalk and the dome are sucked into the mold cavity together with the molten metal, thereby generating casting defects. Thus, to improve the quality of cast products, air and solid metal pieces should be prevented from being mixed with the molten metal.